1. Field of the Invention
The present invention relates to a signal transceiver and adaptive impedance switch circuit, and more particularly, to a signal transceiver and adaptive impedance switch circuit capable of effectively improving return loss when the signal transceiver operates in a power-off state.
2. Description of the Prior Art
Ethernet over Coax (EoC) is a transmission technology in which the Ethernet signals are transmitted over a coaxial cable. The objective of EoC is to connect to the Internet or wideband data transmission utilizing existing cable television infrastructures, which is compatible with existing cable (or satellite TV) broadcast signals, to reach the goal of simultaneously transmission of data signals over the same coaxial cable. Among the EoC methods, the multimedia network standard developed by the multimedia over coax alliance (MoCA) has functionalities of high speed and high quality of service (QoS) which are required for the glitch-free streaming media. According to the multimedia network standard, signals can be sent to each client through the existing coaxial cable, such that the client only needs a signal transceiver to demodulate the signals to obtain services.
Please refer to FIG. 1, which is a schematic diagram of a conventional signal transceiver 10. The signal transceiver 10 includes a connector 100, a band-pass filter (BPF) 102 and a front-end module 104. Usually, the signal transceiver 10 is implemented with a set-top box (STB). The connector 100 connects one coaxial cable, for receiving signals including a MoCA signal, which is transmitted via the coaxial cable. The BPF 102 is utilized for filtering the signal, so as to pass the signal within a frequency band. For example, the range of the frequency band of the MoCA signal provided by the U.S. satellite TV service provider DIRECTV™ is from 475 MHz to 625 MHz. If only the MoCA signal needs to be passed, the frequency range of the BPF 102 should be set from 475 MHz to 625 MHz. The front-end module 104 is utilized for demodulating the signal through the BPF 102. In general, the front-end module 104, which is usually integrated into an integrated circuit (IC), includes circuits such as a transmitter-receiver, a power amplifier and an attenuator, etc.
Please refer to FIG. 2A and FIG. 2B, which are schematic diagrams of the return loss between the connector 100 and one coaxial cable (not shown) connected to the connector 100 within a frequency band of 475-625 MHz when the signal transceiver 10 operates in a power-on and power-off state, respectively. By comparing FIG. 2A and FIG. 2B, it can be seen that within the frequency band of 475-625 MHz, the minimum return loss of the signal transceiver 10 in the power-off state is nearly 7.6 dB, which is 3.4 dB lower than in the power-on state (nearly 11 dB). As can be seen from the above, if the signal transceiver 10 operates in the power-off state, the system may encounter performance degradation due to over-low return loss.